It is our goal to learn the pattern of function of the mammalian left and right ventricles, aorta, arteries, capillaries and veins; to determine the geometric, kinematic and dynamic similarity criteria which define the mammalian circulatory system (ventricles, aorta, arteries, capillaries and veins) of all mammals from the 3 gram shrew to the 100 ton whale; and to write mathematical equations describing the hemodynamics of the ventricles, aorta, arteries, capillaries and veins. Plastic and silastic casts of the arterial, capillary and venous systems will be prepared in mice, rats, rabbits, dogs, goats, cows and horses and measurements made of the diameter and length of the widest variety of arterial and venous segments. (An "arterial segment" is the length of vessel between the points where two branches come off.) The pressure- drop and velocity of flow in these arterial segments will be measured in the above wide variety of mammals. The results will be analyzed from the standpoint of the Principle of Similarity and Dimensional Analysis attempting to write mathematical equations describing the dynamics of flow in terms of the pressure coefficient, Reynolds number and ratio of length to diameter of vessel segments. In addition studies will be carried out in dogs to determine what fraction of the total potential energy developed by the left ventricle is dissipated as kinetic energy in ejecting the stroke volume; to determine left ventricular volume by means of the indicator-dilution technique in very large and very small dogs; and to carry out experiments concerning the question of "aortic mixing" as a source of error in the measurement of left ventricular volume by indicator-dilution.